The present invention generally relates to optical recording apparatuses, and more particularly to an optical recording apparatus for recording an image on a recording medium by means of finely focused optical beams produced by a laser diode array.
Laser printers combine the xerographic technique of image recording with the laser scanning technique for writing images by a finely focused laser beam. Thus, the apparatus has distinct advantages such as high speed operation, high quality printout, capability of recording on ordinary recording sheets, quiet operation, and the like, over other printers such as wire dot printers, thermal printers, ink-jet printers, and the like. Thus, use of laser printers is spreading rapidly in the output device of computers and digital copiers. In relation to the laser diodes, various studies are being made for improving the performance thereof.
For example, Japanese Laid-open Patent Publications 59-19252 and 1-106486 describe a conventional optical image recording apparatus wherein finely focused laser beams, produced by a laser diode array, are deflected to scan a recording surface of a recording medium. The apparatus of the reference thereby monitors the output optical power of the laser diodes during the fly-back interval of the line-by-line scanning process, by means of a back-beam detector or monitor photodetector formed internally to the package of the laser diode array. There, the laser diodes included in the laser diode array are activated one by one in the fly-back interval for monitoring the output optical power of the individual laser diodes. The laser diodes, in turn, are controlled in response to the output of the back-beam detector such that an output optical beams are obtained with a controlled output optical power.
Japanese Laid-open Patent Publication 62-273862 describes another optical recording apparatus that uses a laser diode array, wherein the drive current of the laser diodes is subjected to a periodical resetting process with an interval smaller than the time constant of thermal coupling between the laser diodes. Thereby, the variation of optical output caused by the thermal coupling of laser diodes (a thermally induced interference between laser diodes) is reduced and the output optical power is held substantially constant.
Further, Japanese Laid-open Patent Publication 1-155676 describes an optical recording apparatus that uses back-beams emitted in a backward direction from laser diodes included in a laser diode array, wherein the back-beams are detected by corresponding photodiodes for controlling the output power of the individual laser diodes. In the device of the reference, there are provided shading parts in the receiving surface of the photodiodes for eliminating cross-talk between different back-beams.
FIG. 1 shows the construction of the scanning system used in a conventional laser printer.
Referring to FIG. 1, a laser diode 1 produces a laser beam modulated in response to a modulation signal, and the laser beam thus produced is directed to a rotary mirror 3 after passing through a collimator lens 2. At the rotary mirror 3, the optical beam is deflected and forms a tiny optical spot on a photosensitive drum 5 after passing through a focusing lens (a f-.theta. lens) 4. The optical spot thus formed scans the surface of the photosensitive drum 5 in response to the rotational motion of the rotary mirror 3, wherein the photosensitive drum is rotated simultaneously to the rotary mirror 3. Thereby, an electrostatic latent image is formed on the photosensitive drum 5. In addition, there is provided a photodetector 6 at a position offset from the effective scanning range of the optical beam in which the writing is made on the photosensitive drum 5, for controlling the origin or reference position of the recording that is made on the photosensitive drum 5.
In such a laser printer, the photosensitive drum 5 is rotated about a rotary axis thereof with a speed of typically about 500 mm/sec. In correspondence to this, the rotational speed of the rotary mirror 3 is given according to EQU R=V.sub.o .times.DPI.times.60.times.(25.4.times.N) (1)
wherein V.sub.o represents the speed of the drum 5 represented in the unit of mm/sec, DPI represents the number of dots to be recorded per inch, and N represents the number of reflecting surfaces formed on the polygonal rotary mirror 3. Typically, the parameter DPI is set to 300-400, and the parameter N is set to 6-10. In the case when the parameters are set as V.sub.o =500, DPI=300, and N=8, it will be understood that the rotary mirror 3 has to rotate with a speed of as much as 44291 rpm.
However, such an extremely fast rotational speed of the mirror 3 poses a difficult problem with respect to the bearing mechanism that provides the axial support of the mirror 3. For example, conventional ball bearing mechanism cannot be used for supporting the axle revolving in such a high speed. Thus, one needs special bearing mechanism such as fluid bearing or magnetic bearing, while use of such special bearing inevitably increases the cost of the apparatus. Further, associated with the use of such a high rotational speed for the rotary mirror 3, one has to use a very high modulation frequency for modulating the laser diode. Thereby, a high data transfer rate is required for transferring data from host machine to the laser control circuit that control the laser diode, and adaptation of the system in conformity with such a high data transfer rate also increases the cost of the apparatus.
In order to meet the stringent requirement of such a high speed operation, a system is proposed wherein the optical source is configured to produce a plurality of laser beams in the form of a laser beam bundle, and wherein a single rotary mirror is used to deflect such a laser beam bundle. Thereby, a plurality of lines are recorded by a single scanning of the laser beam bundle. When one can have M laser beams in the laser beam bundle, the rotary speed of the rotary mirror 3 and the modulation frequency of the laser diode can be reduced by a factor of 1/M, and the cost of the apparatus decreases dramatically.
In order to form such a laser beam bundle, laser diode arrays that includes a number of laser diodes on a common chip is used, wherein the individual laser diodes in the array can be modulated independently from each other. The laser diode array having such a construction is preferable in view point of increased accuracy in the pitch of laser beams on the photosensitive drum. It should be noted that the pitch of the laser beams on the photosensitive drum is determined by the device structure and is affected little by the thermal deformation of the optical system. By using the laser diode array to form an optical beam bundle, one can also simplify the structure of the optical system such that the optical system includes only one set of optical elements such as one collimator lens and one focusing lens.
FIG. 2 shows an example of a laser diode array 7 wherein three laser diodes 8a, 8b and 8c are arranged to form an array. There, it should be noted that the laser diodes 8a, 8b and 8c are aligned with each other and driven independently to produce optical beams modulated independently from each other.
When operating the laser diode array 7 such that the individual laser diodes are modulated independently, there is a tendency that the operation of the individual laser diodes is influenced by the on-off operation of other laser diodes via the thermal interference effect. It should be noted that such a thermal interference effect causes a change of temperature in the individual laser diode device. As the drive current-output optical power characteristics of the laser diodes 8a, 8b and 8c are influenced significantly by the operational temperature, the optical output of the laser diodes 8a, 8b and 8c tends to fluctuate significantly due the thermal interference, even when the laser diodes are driven by a constant drive current. According to the experiment, it was shown that the time constant of thermal interference (thermal coupling time constant) has a value of several milliseconds when the devices are separated from each other by a distance of 100 .mu.m. However, the value of the time constant increases to several hundred microseconds when the separation between the adjacent devices is decreased to 50 .mu.m.
The use of laser diode array as shown in FIG. 2 in the optical recording apparatus raises another problem related to the optical system in that the laser beams forming the laser beam bundle produced by the laser diode array 7 tend to pass through the optical elements such as collimator lens and focusing lens, at a part offset from the optical axis, except for one optical beam that is aligned on the optical axis. Thereby, the optical beams generally experience aberration and the shape of the beam spot deforms inevitably. Further, there is a tendency that the scanning line may be curved due to the aberration of the optical system.
Such a problem of aberration can be minimized by reducing the separation between the laser diodes in the laser diode array 7. However, excessive reduction in the separation between the laser diodes invites the problem of thermal interference of the laser diodes and the optical output of the laser diode changes inevitably. It should be noted that the scanning in laser printers is achieved generally with a period of several milliseconds to several hundred microseconds, and the operation of the laser diodes with such a scanning period inevitably causes the problem of thermal interference in view of the result of experiment mentioned above. In the construction of the Japanese Laid-open Publication 62-273862, for example, it is difficult to control the thermal time constant below the scanning period, and the problem of the output fluctuation by thermal interference appears.
In order to compensate for such a variation of the optical output, the conventional optical recording apparatuses employ optical detectors such as photodetector for monitoring the output power of the laser diodes and for correcting any variation of the output optical power.
FIG. 3 shows the construction of the Japanese Laid-open Application 1-155676, wherein there are provided photodetectors 9a, 9b, . . . in number corresponding to the number of the laser diodes forming a laser diode array 10, wherein the individual photodetectors 9a, 9b, . . . are separated from each other by a shade element 11 provided between adjacent photodetectors. Thereby, the shade element 11 eliminates the problem of cross-talk between divergent laser beams produced by the laser diode array 10. In the construction of FIG. 3, however, the overlapping of the divergent beams increases excessively when the separation between the laser diodes is reduced in the laser diode array 10, and the elimination of cross-talk between different laser beams is no longer effectively achieved. Thus, there is a problem in that the device of the reference is not sufficient for providing high quality printout image due to the variation of optical power and cross-talk between the photodetectors.